The removal of organic substances from process and waste streams poses a significant challenge for many industries. This challenge has further been compounded by the increasing rigorous quality control restrictions imposed on manufacturing industries. Depending on the nature of the industry, organic substances in industrial process and waste streams can be a complex mixture of different compounds ranging from simple low molecular weight hydrocarbons such as alcohols, aldehydes, ketones, carboxylic acids, low molecular weight fatty acids to high molecular weight hydrocarbons such as fulvic and humic acids. Organic substances in industrial waste effluents may contain nitrogen, chloride, and sulfur and some effluents contain process chemicals which need to be recycled, e.g. black liquor from Bayer process and pulp mill.
Conventional methods for the treatment of organic-containing industrial effluents include biological and physical treatments, incineration and wet air oxidation. Biological treatment is the most widely used method due to its simplicity and low cost. However, the microorganisms used for this process are effective for only low organic content wastes. Thus for high organic content wastes, methods such as wet air oxidation become very attractive.
It is known from the prior art that wet air oxidation is a process in which organic substances in aqueous streams are oxidised by an oxidant. Depending on reaction conditions and the type of organic compound to be oxidised, both non-catalytic and catalytic wet oxidation can be used to convert organic substances into CO2 and biodegradable low molecular substances, such as mono or dicarboxylic acids.
Non catalytic wet air oxidation is a well-established technique for waste water treatment (Swed. Pat. 34941,1911). It entails the liquid phase oxidation of organics and oxidisable inorganics at elevated temperatures (up to 320° C.) and pressures (up to 20 MPa) using a gaseous source of oxygen. The extreme conditions employed often result in severe technical difficulties as well as increased capital cost.
Catalytic wet oxidation is a promising alternative technique, which can operate at lower temperatures and pressures. Catalytic wet oxidation can be carried out by means of homogeneous or heterogeneous catalysis. There are a number of homogeneous catalytic systems reported which effectively oxidise organic substances from aqueous streams. Examples of such catalytic systems are CuSO4 and Cu(NO3)2. (Lei et al., Wat. Sci. Tech., Vol. 35, No. 4, 1997, pp 311-319; Lin and Ho, J. Environ. Eng., September, 1997, pp 852-858). However, the need for down stream processing to remove the spent catalyst is a distinct disadvantage making homogeneous technology commercially infeasible.
Heterogeneous catalysis appears to be a better alternative. In heterogeneous oxidation catalysis, the catalytic activity is attributable to surface oxygen available on the solid catalyst. A good catalyst is characterised by high surface oxygen availability and fast oxygen transfer ability. Depending on the application, the catalyst may be in the form of a powder, pellets or a monolith.
Traditionally, catalytic wet air oxidation is carried out in slurry or trickle bed reactors. In slurry reactors, the catalyst has to be recycled continuously to the reactor. Unfortunately, separation of the fine catalyst particles can be troublesome, costly and time-consuming. To overcome this, a trickle bed reactor with catalyst in the form of pellets or monoliths may be used. Catalysts in the form of pellets are well known as are techniques for forming such pellets. In the monolithic system, catalytic materials are deposited onto the monolith support in the form of a very thin layer. The monolith support can be in the form of honeycombs with parallel open channels. Reactants may flow through the monolithic channels. In this reactor configuration, the need to filter and recycle catalyst back to the reactor is eliminated.
Industrial interest has stimulated numerous investigations into catalysts for wet oxidation of organic substances from process and waste streams. Although a number of catalytic systems have been reported in the open and patent literatures, most of the catalytic systems require severe conditions, namely high pressures and high temperatures, in order to achieve significant oxidation rate (U.S. Pat. No. 4,268,399-A; Eur. Pat. 90313238.9; Lin and Ho, J. Environ. Eng., September, 1997, pp 852-858; Zhang and Chuang, Ind. Eng. Chem. Res., 37, 1998, pp 3343-3349). In some catalytic systems, strong oxidants such as ozone and H2O2 are required (Centi et al., Catal. Today, 55, 2000, pp 61-69; Gulyas et al., Wat. Sci. Tech., Vol 32, No 7, 1995, pp 127-134).
Accordingly, it is an object of the present invention to overcome, or at least alleviate, the difficulties presented by prior art.